1. Field of the Invention
The present invention relates to a piezoelectric actuator drive method, a piezoelectric actuator drive device, an electronic timepiece, an electronic device, a control program for a piezoelectric actuator drive device, and a storage medium.
2. Description of Related Art
Since piezoelectric elements have excellent response and efficiency in converting from electric energy to mechanical energy, various piezoelectric actuators that utilize the piezoelectric effects of piezoelectric elements have recently been developed. These piezoelectric actuators are being applied to the fields of piezoelectric buzzers, inkjet heads in printers, ultrasonic motors, electronic timepieces, portable devices, and other such electronic devices.
Size and thickness reduction is in demand for piezoelectric actuators used in these fields; therefore, piezoelectric actuators have been proposed that include a thin rectangular piezoelectric element and a plate-shaped oscillator, wherein the piezoelectric element is stretched in the longitudinal direction and caused to perform longitudinal oscillation by supplying voltage to the piezoelectric element, and bending oscillation is mechanically induced by the longitudinal oscillation.
In such a piezoelectric actuator, creating both longitudinal oscillation and bending oscillation in the oscillator allows the region in contact with the drive object of driving in the piezoelectric actuator to move in an elliptical orbit. Thus, the piezoelectric actuator achieves highly efficient driving with a small and thin configuration.
Conventional art for driving a piezoelectric actuator with the previously described configuration is disclosed in Japanese Laid-open Patent Application No. 2002-291264 (paragraph nos. [0057] to [0075], hereinafter referred to as Patent Literature 1).
Patent Literature 1 has a configuration wherein the phase difference in oscillation is determined based on a signal detected from the piezoelectric element, and the drive pulse supplied to the piezoelectric element is controlled in constant increments per pulse to obtain a drive frequency (target value) with the optimal phase difference. In Patent Literature 1, a frequency in a specific range is adopted for the drive signal during locking, a constant value within this range is used as the target value, and phase control is performed in a specific range FL of this target value, as shown in FIG. 21.
In Patent Literature 1, the phase difference alone is used to control the drive signal sent to the piezoelectric element, and it is not easy to control the phase difference.
In other words, the frequency needed to achieve highly efficient drive conditions wherein the phase difference reaches the target value is adjusted within a specific time period, and the controlled variable is made constant for each pulse (for each specific time period).
Therefore, when the controlled variable per specific time period is large, the frequency is continuously adjusted after reaching the vicinity of the target value, so the drive frequency becomes increasingly erratic and much time is required for the frequency to converge on the target value.
Depending on the situation, the controlled value may overshoot and the piezoelectric actuator may be controlled at a drive frequency far distanced from the target value (see Px in FIG. 21).
Conversely, when the controlled variable per specific period of time is small, too much time is required after turning on the power source to achieve highly efficient drive conditions wherein the vicinity of the target value is reached, and the amount of electric power consumed increases accordingly.
An object of the present invention is to provide a piezoelectric actuator drive method wherein the time required to achieve highly efficient drive conditions is shortened to reduce power consumption and to perform stable control, to provide a piezoelectric actuator drive device, to provide an electronic timepiece and an electronic device that contain this piezoelectric actuator, to provide a control program for a piezoelectric actuator drive device, and to provide a storage medium that stores this control program.